Structure is known in which a main catalytic converter at the underbody of a vehicle is disposed downstream of an exhaust system of the vehicle's internal-combustion engine. With this arrangement, sufficient exhaust purification cannot be expected after starting the engine under cold conditions until the catalytic converter is activated with its temperature raised. In contrast, if the catalytic converter were arranged upstream relative to the exhaust system; that is, closer to the engine, durability would be reduced due to thermal deterioration of the catalyst.
Therefore, as disclosed in Japanese Laid-Open Patent Application No. H05-321644, proposals have been made for an exhaust system in which an exhaust bypass is provided parallel to the upstream portion of the main exhaust path in which there is a main catalytic converter, and another catalytic converter is provided in the exhaust bypass, the exhaust gases being guided to the exhaust bypass immediately after starting under cold conditions by means of a switching valve that switches between the main exhaust path and the exhaust bypass. With this structure, the exhaust bypass catalytic converter is located upstream relative to the main catalytic converter, and exhaust purification can be commenced at an earlier stage because catalyst activation is carried out at a relatively early stage.